This invention relates generally to a miniature motor, such as a stepping motor, used for printers and facsimile machines, and more particularly to a miniature motor in which only a small number of component members are required, the axial relative movement of the rotor with respect to the housing is controlled, whereby preventing mechanical noise.
Stepping motors, one type of miniature motors, have heretofore been widely used as the drive gear for information processing equipment and various other equipment. FIG. 1 is a longitudinal sectional view illustrating the essential part of an example of stepping motor of a conventional type. In FIG. 1, numeral 1 refers to a housing made of a metallic material, such as mild steel, and formed into a bottomed hollow cylindrical shape, on the inner circumferential surface of which yokes 2 made of a ferromagnetic material and formed into a ring shape, coil bobbins 3 made of an insulating material and formed into a ring shape, and coils 4 wound on the coil bobbins 3 are provided. Numeral 5 refers to an end plate to be fitted to an open end of the housing 1.
Numeral 6 refers to a rotor made of a permanent-magnet material, such as ferrite, and formed into a cylindrical shape and having a plurality of magnetic poles extending axially, which are disposed radially on the outer circumferential surface thereof. Numeral 7 refers to a shaft fixedly fitted to the center of the rotor 6 and rotatably supported by bearings 8 provided on the housing 1 and the end plate 5. Numeral 9 refers to washers made of a resin material and formed into a ring shape, and fixedly fitted to the shaft 7 in such a manner that the washers 9 are interposed between an end face of the rotor 6 and the inner end face of the housing 1, and between the other end face of the rotor 6 and the inner end face of the end plate 5. A spring washer 10 made of a metallic material having spring properties is interposed between the rotor 6 and the end plate 5 in a state that the spring washer 10 is sandwiched between two washers 11 and 12.
With the aforementioned construction, as current is fed to the coils 4 via a connector 13 and a printed-circuit board 14, the yokes 2 are excited by a combination of two-phase currents flowing in the coils 4, causing the rotor 6 having a plurality of magnetic poles disposed on the outer circumferential surface thereof to rotate in short and uniform angular movements. Thus, power is transmitted to driven equipment in accordance with the short and uniform angular movements. Since the spring washer 10 is disposed between the two washers 11 and 12, the rotor 6 is prevented from being vibrated axially and mechanical noise is therefore prevented from being generated.
However, the prior-art means having the aforementioned construction for preventing the rotor 6 from being vibrated or moved axially has the following problems. The force pushing the rotor 6 towards the washer 9 on the side of the housing 1 is dependent on the amount of axial deflection of the spring washer 10. In order to ensure the optimum amount of deflection of the spring washer 10, therefore, it is necessary to finely adjust the axial free dimensions between the washers 9 and 12 with respect to the axial dimensions between the inner end faces of the housing 1 and the end plate 5.
To this end, a plurality of washers 9, 11 and 12 having different thicknesses have to be prepared in advance. In addition, a measuring tool or inspection tool is also needed to confirm whether the spring washer 10 has the optimum amount of deflection. All this makes the assembly work of miniature motors extremely complex.
Furthermore, even after the troublesome fine adjustment described above, variability in the resiliency of the spring washer 10 cannot be avoided. Consequently, the state of mutual friction and engagement tends to be unstable among the bearing 8, the washer 9 and the rotor 6 on the side of the housing 1, and among the washer 11, the spring washer 10, the washer 12 and the end plate 5 on the side of the end plate 5. As a consequence, the resulting loss of torque also tends to be variable. This adversely affects the output torque transmitted from the shaft 7.